How is an architecture composed of a second model that validates the first one called in machine learning?
Not in terms of models, but there is a terminology called 'Hierarchical learning', wherein if your model has a task to classify disease, then, If it detects a presence of a disease (disease/ no disease), then it proceeds to further classify a disease(class A/B/C/...). Else it does not proceed.
This technique of hierarchical learning is very common amongst supervised learning tasks.

Now according to your question, you have two models and I assume that they have different tasks and provide a binary outcome(yes/no). Here, you can call it as 'Multitask learning', where the output of task1 is given to task2 for processing.
If task1 detect the presence of disease, then task2 classifies disease into various classes / or segment it / localize it etc.